Fixes rvm_prompt_info() in lib/rvm.zsh

Fix comes into play when ZSH_THEME_RVM_PROMPT_OPTIONS actually contains a value.

Let's say we have this:
``` bash
ZSH_THEME_RVM_PROMPT_OPTIONS='i v'
rvm_prompt=$($HOME/.rvm/bin/rvm-prompt ${ZSH_THEME_RVM_PROMPT_OPTIONS} 2>/dev/null)
echo $rvm_prompt # => i v
```

The order in which variables are resolved seems to be the key here... The command substition sees only the second statement, which is the string 'i v', thus I think it's needed to eval the whole call to rvm-prompt first, including its options.

I am happy for suggestions if there is another way to solve this, I am not sure - if not, the eval works in any event.